The present application claims priority to Japanese Application No. P2000-086687 filed Mar. 27, 2000, which application is incorporated herein by reference to the extent permitted by law.
The present invention relates to an optical recording medium, in particular, it relates to a high density optical recording medium.
Write-once discs using an organic pigment have been standardized as a CD-R (compact disc-recordable), or the like, and they have already been used widely.
In the development of the organic pigment, various aspects, such as the optical characteristics, and the thermal characteristics should be optimized.
Among them, as to the optical characteristics, for example, in the case of the CD-R, and the DVD-R (digital versatile disc-recordable), since they each need to be produced so as to match the standard of the reading dedicated disc of the CD and the DVD, they should be designed so as to have the refractive index of 70% or more in the CD-R, and 60% or more in the DVD-R with respect to the wavelength of a light source. Therefore, it is realized by providing a metal reflection film with a high refractive index, and using an organic material with a low imaginary part of the complex refractive index (absorption coefficient k). Furthermore, in order to have a sufficient modulation degree, the real part of the complex refractive index (refractive index nb) before recording needs to be high.
Moreover, as to the absorption coefficient k, there is another reason. That is, the heat conductivity of an organic pigment is, in general, about 0.2 J/mxc2x7sxc2x7K, which is extremely lower than a recording material of a photomagnetic disc. Therefore, the temperature can easily be raised at the time of recording, and thus the absorption coefficient k should be provided as low as possible (see the official gazette of Japanese Patent Application Laid-Open (JP-A) No. 7-272314, and the official gazette of Japanese Patent Application Laid-Open (JP-A) No. 7-282465).
As mentioned above, media matching the standard of the ROM (read only memory), such as the CD, and the DVD have conventionally been developed. In contrast, in consideration of the circumstances that the reflectance of a rewritable medium such as a phase change material including the CD-RW (CD-rewritable) cannot be substantially the same as that of the ROM as the principle, a low reflectance is included in the standard for the next generation high density optical discs as the basic standard.
In order to realize such an organic pigment disc with a low reflectance, a problem arises in the above-mentioned conventional configuration.
That is, the reflectance can hardly be restrained at a low level in such a configuration with a reflection film, and when the reflectance is lowered, utilizing the multiple interference, or the like, the absorption ratio of the light is raised extremely so as to generate a problem of heat.
This is partly because dispersion of heat by the reflection film is insufficient due to the low heat conductivity of the organic pigment material.
As a medium configuration coping therewith, that realizing the same reflectance as that of the phase change optical recording medium without using a reflection film for ensuring a sufficient modulation degree has been proposed (for example, see the official gazette of Japanese Patent Application Laid Open (JP-A) No. 6-76359).
Recently, with development toward a shorter wavelength of a light source, a blue violet semiconductor laser (wavelength 380 nm to 450 nm) is to be used practically. By use of the short wavelength laser, a drastically high density of an optical recording medium can be achieved.
However, the conventional optical recording medium with an organic pigment cannot cope with such a short wavelength light source. Moreover, a problem of difficulty in enduring a high energy density in a spot due to the minute laser beam spot by the short wavelength of the laser, and the high numerical aperture (high N. A.) of the optical system is involved.
That is, for example, with the commercialization of a pickup for optical recording and/or optical reproduction (hereinafter referred to simply as an optical pickup) by an optical system of a 0.8 or more N. A. (for example, see P. 131 of K. Osato et al., Digest of Optical Data Storage (Aspen, Colo., 1998), or the like), a measure for a higher energy density is called for.
Moreover, the recording area of the CD-R is provided only in the groove. In this case, the groove width is to be set narrower than the land width. A laser beam irradiation with respect to the groove is directed also over the land part. In the CD-R, as mentioned above, the reflectance thereof is set to be high, that is, the reflectance is set at 60% or more regardless of existence of record so that by detecting reflections from both land and groove at the same time, the effective reflectance change, that is, the recorded information is read out, utilizing the interference effect generated from the displacement of the reflected light phases from each of them by the phase modulation method. Therefore, in the CD-R, the so-called land groove recording mode for recording on both land and groove, cannot be adopted.
Furthermore, on the other hand, a high density can be realized also by providing multiple layers in the information retaining layer. (see P. 197 of K. Kurokawa et al., Digest of International Symposium on Optical Memory/Optical Data Storage (Koloa, Hi., 1999)).
In order to realize this, it is necessary to restrain the reflectance of each layer at a low level and at the same time to make the transmittance high. However, since a recording layer of a rewritable type optical disc made from an inorganic material, in general, has a high absorption coefficient and a low transmittance, it has been difficult to realize multiple layers comprising three or more layers.
The present invention is to provide an optical recording medium for solving the above-mentioned problems.
That is, the present invention relates to a high density optical recording medium, capable of realizing the complete compatibility with a phase change recording medium with respect to an optical pickup or a drive device, using a short wavelength light for achieving a high recording density.
Moreover, the present invention is to realize a further high density recording medium by enabling adoption of the so-called land groove recording mode, capable of recording on both land and groove.
Furthermore, according to the present invention, since the reflection film as in the conventional organic pigment disc is not used, the reflectance can be restrained as well as a high transmittance can be obtained at the same time so that further higher density can be realized by providing multiple layers.
That is, the present invention provides an optical recording medium configuration for directly reading out the change of the reflectance without using the phase modulation method.
An optical recording medium according to the present invention is an optical recording medium having a recording film of a single layer or multiple layer structure on a substrate, wherein the recording film is mainly made of an organic material and decomposed after absorbing a laser beam to have a refractive index change, and reflectance of the optical recording medium with respect to a reproducing laser beam having a wavelength of 380 nm to 450 nm is 15% to 25% before the decomposition, and 0% to 10% after the decomposition.
That is, an optical recording medium according to the present invention directly reads out the change of the reflectance without using the phase modulation method as mentioned above.